According to well-placed sources, Danger Mouse has been mixing the album at Electric Lady and the band has recently joined in. U2 is apparently finished recording, and hasn't done any new recording at Electric Lady. There was a playback last week, which is apparently what Blackwell tweeted about.

Album mixing typically happens when recording is finished. It essentially involves putting all the pieces together, adding effects, making sure sound levels are correct, etc. That said, U2 is legendary for continuing to work on songs even during the mixing process. As Steve Lillywhite said in an interview last year, "They'll write the song after you've mixed it!"